1. Field of the Invention
This invention relates generally to radio frequency power delivery, and more particularly to the detection and avoidance of arcing in radio frequency powered plasma processes.
2. Brief Description of the Prior Art
Radio frequency (RF) powered plasma processes are commonly used in the manufacture of semiconductors, flat panel displays, data storage devices, and in other industrial applications. While RF power supplies are typically well protected against sudden changes in load impedance, they generally are not designed to detect and respond to changes in plasma impedance caused by arcing within a process chamber. As a result, an RF power supply may continue to feed energy into incipient arcs that develop within a plasma process, which in turn may cause serious damage to the surface of a workpiece or even to the processing equipment itself.
In DC powered plasma processes, the problem of arcing has long been studied, particularly in reactive sputtering applications. In a reactive sputtering process, arcing often results from charge buildup and eventual electrical breakdown on the surface of dielectric films deposited on the sputtering target or chamber walls. Problems of arcing in DC plasma processes have been addressed by through the use of sophisticated arc handling systems capable of detecting arcs and of employing any number of techniques to mitigate their severity, such as temporarily interrupting power or reversing the polarity of output voltage. In critical applications, the time during which the output voltage is removed is taken into account to adjust processing time so that the total energy delivered to the plasma is controlled and limited. In DC systems, it has also long been recognized that pulsing the DC output or reversing the output polarity at a certain repetition rate and duty cycle can reduce the tendency of arcs to develop.
RF power has been seen as an alternative technology that may be used to sputter an insulator directly while avoiding altogether the arcing problems in DC sputtering processes. Only recently has it been recognized, however, that occasional arcing occurs in RF processes as well, and that for sensitive film properties or geometries this RF arcing can be equally as damaging. Arcing in RF powered systems may result from charge buildup across gate-electrode patterns on semiconductor wafers or upon polymer coatings on chamber surfaces. Other factors include defects in the reactor or chamber hardware, degradation of the protective chamber anodization layer, differences between the electrical potential across tool parts, or even simply the magnitude of the RF power being applied. In any event, handling and avoidance of arcing requires the capability of both rapidly detecting the onset of an arc and rapidly interrupting or removing the output power so as to reduce the energy delivered into the arc.
In one approach, arc detection and avoidance in RF systems has been based upon establishing a predetermined threshold of a power delivery parameter, such as reflected power. The occurrence of an arc is inferred from a sudden rise or spike in reflected power that exceeds the predetermined threshold. This approach is not effective, however, while the power transfer of the system is being tuned, i.e., before the reflected power of the system has been brought to a steady state value that is below the predetermined threshold. The threshold approach is also limited in that arcing in an RF processing application does not always lead to an increase in reflected power. Depending on the state of the match network, an arc may in fact reduce the reflected power, and therefore not trigger an arc detection in a simple threshold circuit.
Another approach to RF arc detection correlates the derivative, or time rate-of-change, of a power delivery parameter to an arcing condition. Some RF arcs may develop slowly, however, over a period of 1 microsecond or longer, and may therefore go undetected by a derivative detector. Moreover, the derivative detector has increasing gain with frequency up to a point where practical limitations restrict the bandwidth. As a result, the derivative detector becomes more sensitive to noise at higher frequencies of operation.